Stripping of coated titanium electrodes

ABSTRACT

A METHOD FOR STRIPPING THE COATING FROM AN ELECTRODE COMPRISING A TITANIUM SUPPORT AND A COATING COMPRISING A PLATINUM METAL OXIDE THEREON WHICH COMPRISES IMMERSING THE ELECTRODE IN AN AQUEOUS MIXTURE COMRPISING AT LEAST ONE STRONG MINERAL ACID, OTHER THAN NITRIC ACID, WITH HYDROFLUORIC ACID AND/OR A PRECURSOR COMPOUND THAT WILL REACT WITH THE MINERAL ACID TO GIVE HYDROFLUORIC ACID, AT A TEMPERATURE AND FOR A TIME SUFFICIENT FOR THE COATING TO BECOME DETACHED FROM THE TITANIUM SUPPORT.

United States Patent S TR IPPING 0F COATED TITANIUM ELECTRODES John Hubert Entwisle, Anthony Scrutton, and Keith Graham Moss, Runc0rn,-England, assignors to Imperial Chemical Industries Limited, London, England No Drawing. Filed May 27, 1971, Ser. No. 147,608

Claims priority, application Giggt Britain, June 4, 1970,

9 Int. Cl. C23g N02 US. Cl. 134-3 8 Claims The present invention relates to stripping of coated titanium electrodes for .re-coat-ing. More particularly it relates to a method for the removal'of a coating compris- "ing one or more platinum metal oxides from a titanium support which carries the coating, in'a manner which renders the titanium surface suitable for re-coating and without excessive loss oftitanium fromthe support.

It has recentlybeen proposed to employ as electrodes, particularly as anodes in cells electrolysing aqueous solutions of alkali metal chloride, a combination of a filmforming metal support and a .coating thereon comprising oxides of one or more of the platinum group metals in admixture with an oxide of a film-forming metal. (See for instance British patentspecification No. 1,147,442 and Belgian patent specification No. 710,551.) In electrodes of this type the film-forming metal of both the support and the oxide coating mixture is most suitably titanium.

Electrodes of the aforesaid type have advantages over prior art electrodes when empolyed as anodes in the electrolysis of alkali metal chloride solution in that they have both low chlorine overpotential characteristics and high resistance to electrochemical attack in use.

Although the electrodes have avery low wear rate in the cell, their useful lives are not infinite and in time they must be replaced or re-coated in order to maintain optimum performance. When the electrodes are taken out of service they still retain an appreciable amount of the original coating and this must be removed if the titanium support is to be used as a support for a new coating. This presents the problem of removing the old coating cheaply and easily without at the same time removing an unacceptable amount of titanium metal from the support. Also, the method used for removing the old coating should desirably leave the surface of the titanium support in a suitable condition for re-coating without (further treatments, such as the conventional etching treatments, which necessarily remove further amounts of titanium metal.

The present invention provides a solution to these problems of stripping oxide-coated titanium electrodes of the aforesaid type.

According to the present invention we provide a method for stripping the coating from an electrode comprising a titanium support and a coating comprising a platinum metal oxide thereon which comprises immersing the electrode in an aqueous mixture comprising at least one strong 3,761,313 Patented Sept. 25, 1973 mineral acid, other than nitric acid, with hydrofluoric acid and/or a precursor compound that will react with the mineral acid to give hydrofluoric acid, at a temperature and for a time sufiicient for the coating to become detached from the titanium support.

The preferred mineral acid is hydrochloric acid or sulphuric acid.

When hydrochloric acid is used it is preferably such that it comprises 20% to 40% hydrogen chloride by weight of the total of hydrogen chloride and water.

When sulphuric acid is used it is preferably 25%. to 100% by weight of the total of sulphuric acid and Water.

The hydrofluoric acid is preferably present in a concentration of 0.01% to 8% hydrogen fluoride by weight of the mixture, particularly 0.02% to 4%. If a compound is used instead to react with the hydrochloric or sulphuric acid to give the hydrofluoric acid its concentration should be such that it will give the desired hydrofluoric acid concentration. Usually, the precursor is a fluoride which is preferably soluble in hydrochloric or sulphuric acid. Examples of such compounds include ammonium fluoride which is particularly preferred and alkali metal fluorides, e.g. sodium or potassium fluoride.

The temperature of the mixture in which the stripping is carried out may be for example from 25 C. to 100 C., preferably 60 C. to 100 C.

The time of immersion is not critical but generally the immersion need be no longer than one hour.

The electrodes to be treated in accordance with the invention may have coatings in which the whole of the platinum metal content is in the form of platinum metal oxides. The method of the invention is, however, also applicable to coatings wherein a minor amount of the platinum metal content is in the form of free platinum metals, the remainder and major amount being in the form of oxides. The platinum metal content thus defined may consist of any one or more of the metals platinum, iridium, rhodium, osmium, ruthenium and palladium.

The invention is applicable to electrodes in which the coating consists entirely of platinum group metals, mainly in the oxidised state, and also to electrodes in which the coating consists of platinum group metals, mainly in the oxidised state, in association with oxides of other metals, e.g. oxides of the film-forming metals and in particular titanium as well as other film-forming metals including zirconium, niobium, tantalum and tungsten. Other oxides with which the platinum group metal oxides may be associated include tin dioxide, germanium dioxide and oxides of antimony. Specific examples of these types of coatings are coatings of ruthenium dioxide in association with titanium dioxide and coatings of ruthenium dioxide in association with mixtures of tin dioxide and oxides of antimony.

The invention is further illustrated by the following examples on the stripping of coatings consisting substantially of ruthenium oxide in admixture with titanium dioxide on titanium support. The weight of the coatings was in the range 15-20 g./m. of the titanium surface. All percentages are by weight.

EXAMPLE 1 A section from a coated titanium electrode blade having the coating on both faces of the blade was immersed in a solution consisting of 80 ml. of 35% hydrochloric acid; 20 ml. of water and 0.08% ammonium fluoride (which results in an approximate HF concentration of 0.04%) at a temperature of C. After 45 minutes the coating had fallen away from the titanium support leaving a titanium surface suitable for re-coating. The loss of titanium was 9.4%.

3 EXAMPLE 2 Another section from the coated titanium electrode blade as in Example 1 was immersed in a solution consisting of 100 vols. of 35 hydrochloric acid and 0.4% ammonium fluoride (which results in an approximate HF concentration of 0.2%) at a temperature of 85 C.

After 15 minutes the coating had fallen away from the titanium support leaving a titanium surface suitable for re-coating. The loss of titanium was 13.8%.

EXAMPLE 3 EXAMPLE 4 By way of comparison, another section of the coated titanium electrode blade as in Example 1 was immersed in 80 vols. of 35% hydrochloric acid and 20 vols. of water but containing no fluoride. After 13 hours the coating had only been partly removed and the surface was unsuitable for re-coating.

EXAMPLE 5 Another section of a coated titanium electrode blade was immersed in a solution consisting of 30 ml. of concentrated (98%) sulphuric acid, 70 ml. of water and 0.2% ammonium fluoride (which results in an approximate HF concentration of 0.1%) at a temperature of 80 C. After 53 minutes the coating had fallen away from the titanium support leaving a titanium surface suitable for re-coating. The loss of titanium was 8.1%.

We claim:

1. A method for stripping the coating from an electrode comprising a titanium support and a coating comprising a platinum metal oxide thereon which comprises immersing the electrode in an aqueous mixture comprising (1) hydrochloric acid in a range of 20%40%. hydrogen chloride by weight of the total of hydrogen chloride and water or (2) sulphuric acid in a range of 25 %100% by weight of the total of sulphuric acid and water mixed with (3) hydrofluoric acid in a concentration of 0.01% to 8% hydrogen fluoride by weight of the mixture, said immersing being carried out at a temperature and for a time suflicient for the coating to become detached from the titanium support.

2. A method according to claim 1 in which the concentration of hydrogen fluoride by weight of the mixture is 0.02% to 4%.

3. A method according to claim 1 in which the hydrofluoric acid is formed in situ by using a precursor compound selected from the group consisting of ammonium fluoride and alkali metal fluorides.

4. A method according to claim 1 in which the electrode is immersed for no longer than one hour.

5. A method according to claim 1 in which the platinum metal oxide coating is in association with the oxide of a film-forming metal, tin dioxide, germanium dioxide or an antimony oxide.

6. A method according to claim 1 in which the platinum group metal oxide is ruthenium dioxide.

7. A method according to claim 1 in which the stripping is carried out at temperatures of 25 C. to 100 C.

8. A method according to claim 7 in which the temperatureis C. to C.

References Cited UNITED STATES PATENTS 3,666,580 5/1972 Krcml et a1. 156-18 2,942,954 6/ 1960 Thomas 1343 X FOREIGN PATENTS 1,147,442 4/ 1969 Great Britain.

OTHER REFERENCES Durkin, How To Descale Titanium, Metallurgia, vol. 49, N0. 295, May 1954, p. 256.

MORRIS O. WOLK, Primary Examiner US. Cl. X.R. 252-793 

